1. Field of the Invention
This invention relates to a device for heating a confined fluid medium and is particularly related to heating cryogenic fluids, such as liquified petroleum gas to convert it from the liquid to the gas state.
2. Description of the Prior Art
In the past, cryogenic liquids have been heated using natural draft radiant heaters. This type heater has several drawbacks primarily due to the problem of attaining sufficient heat transfer when the source of heat is in radiant form. In most cryogenic applications the process tubes are below the freezing point and a layer of ice is formed on the tubes. Ice is a very poor heat conductor. Thus, the radiant type heater is seldom used at the present time in cryogenic applications.
Convective heaters have been utilized which transfer heat by flowing tempered products of combustion over a bank of processing tubes (usually finned tubes) through which the cryogenic liquid is circulated. The convective heaters are usually of the fan recirculation, jet recirculation, or excess air type. These type heaters have proved more successful than the radiant type; however, each has certain disadvantages and are relatively expensive.
A submerged exhaust heater has been employed in which the hot products of combustion are bubbled through water to create a froth. Heat transfer tubes carrying the cryogenic fluid are submersed in the water. There are two basic problems with the submerged exhaust type heater, that is: (1) the direct contact between the products and water results in contamination of water causing it to become acidic; and (2) since the combustion has to overcome the water head, this type heater usually requires high blower horsepower compared to the other types of heating systems.
The type of heater which has proven most successful is the indirect water vat heater. A burner is fired into a tube immersed in a bath of water. Process coils are immersed in the same bath above the fire tube. The heated water rises to the top and transfers heat by natural convection to the process tubes carrying the cryogenic liquids. While this heater has proven successful, designs up to the present time have not achieved a high level of energy transfer efficiency and, in fact, the typical unit has a thermal efficiency of about 70%.
It is an object of this invention to provide an improved heater for heating cryogenic liquids such as liquified petroleum gas (LPG). This invention is directed towards an improvement in the indirect water bath heater and has the advantages of the water bath heater in that the flue gas is confined to the fire tube. The process flow coils are immersed in the water bath and are never contacted by the flue gas. In addition to these basic advantages, the present invention is a substantial improvement over the state of the art in that thermal efficiencies of as high as 95% can be attained. In these times of high energy cost and the importance of fuel conservation, the present invention is significant in achieving reduced energy consumption.
A more particular object of this invention is to provide an apparatus for heating a cryogenic liquid such as LPG in a manner wherein the heat transfer efficiency, or thermal efficiency, is greatly improved over existing type heaters.
These general objects as well as other and more specific objects of the invention will be fulfilled in the following description and claims, taken in conjunction with the attached drawings.